Snow Avalanches as a Driver of Large Wood Dynamics in Mountain Streams

Snow avalanches are thought to influence large wood dynamics within mountain streams, but studies that explicitly quantify their impact are minimal. In three headwater catchments of the Frying Pan River, Colorado, USA, we found that wood loads in stream reaches intersected by avalanche pathways ranged from 220 to 8,500 m3/ha and were significantly greater than in non-avalanched reaches. Inter-catchment differences in the number of avalanche occurrences were not reflected in control reach wood loads, suggesting that elevated loads may be restricted to the general vicinity of the avalanche pathway. However, the sizable wood volumes in avalanche reaches generated notable geomorphic impacts, indicating that avalanches can create hotspots of in-stream geomorphic complexity and influence the overall habitat mosaic of the stream ecosystem. Loads in avalanche reaches were moreover significantly greater than literature values for similar streams and recruitment processes, emphasizing the importance of snow avalanches as a wood recruitment mechanism. Snow avalanches are thought to contribute downed, dead wood to mountain streams, but the volume and mobility of supplied wood is unknown. Because downed wood is important for many stream processes (such as habitat creation and sediment storage), an increased understanding of the importance of avalanches for wood supply has notable implications for management of mountain watersheds and their ecosystems. Here we study several streams within the Frying Pan River Basin in Colorado, USA, where avalanche paths commonly intersect the streams that flow along steep mountain valley bottoms. We find that the volume of wood within the length of stream intersected by snow avalanches that occurred in Winter 2019 is substantially greater than the wood volumes outside those reaches. We additionally discover that in-stream wood volumes within the avalanche areas in this study are significantly greater than values reported by other studies for similar streams and recruitment processes. Overall, our work further emphasizes that avalanches are important suppliers of wood to mountain streams and calls for future studies to continue to expand understanding of how snow avalanches impact mountain stream ecosystems. In-stream wood loads supplied by avalanches are on the order of 102-103 m3/ha within avalanche-impacted reachesAvalanche-derived wood may have low mobility but generate hotspots of geomorphic complexityLoads in avalanche reaches are significantly larger than literature values for comparable streams and recruitment mechanisms